The present invention relates to the field of circuits for generating and storing DC control voltages.
Presently, there are numerous electronic functions which are controlled in response to a DC control voltage. For example, in a television receiver the contrast, brightness, tint, color and volume characteristics may be controlled by the application of a DC control voltage to an appropriate circuit such as a variable gain amplifier. In these applications, the DC control voltage must be maintained or stored for a reasonably long period, e.g., several hours, to accommodate most practical television viewing situations. Circuits for adjusting a particular function at a receiver itself often include a potentiometer coupled between a source of DC voltage and the circuit which is to have its function controlled. Once the potentiometer is adjusted to provide the desired result, the associated DC control voltage is maintained until the next adjustment.
Recently, remote control systems have been developed which permit a user to control an apparatus from a remote location. A remote control system when used in conjunction with a television receiver is particularly desirable since it permits a viewer to optimize viewing and listening conditions with respect to his location. In many prior remote control systems, motor driven potentiometers actuated in response to an encoded remote control signal have been utilized to adjust a DC control voltage. In remote control systems utilizing motor driven potentiometers the DC control voltage is readily maintained once it is adjusted to the appropriate setting. However, because of their mechanical nature, these arrangements tend to be complex, bulky, expensive and susceptible to malfunction.
In order to avoid the disadvantages of motor driven potentiometers, other prior remote control systems have been developed which are essentially electronic (i.e., solid state) in nature. One such remote control system includes a counter and a digital to analog (D/A) converter, such as a resistive matrix, which generates a DC control signal in response to the contents of the counter. The contents of a counter are increased or decreased in response to an encoded remote control signal until the desired characteristic is reached. Thereafter, the contents of the counter are held constant until the next adjustment. Such a remote control system is described in the RCA manual entitled, "XL-100 Color Television -- The CTC-74 and CTC-81 Chassis", published by RCA Corporation, Indianapolis, Indiana.
In order to reduce the cost of remote control systems, circuits are desired which more simply and directly generate and maintain a DC control signal than the types of systems described above. For example, a circuit in which a voltage storage capacitor can be selectively charged or discharged to develop a DC control voltage and thereafter effectively isolated to prevent leakage currents from changing the stored voltage may be advantageously employed in a remote control system. Of course, circuits utilizing mechanical or electromechanical (i.e., relay) devices for selectively charging and discharging a capacitor are known. Because of the extremely high "off" (i.e., open) resistance of mechanical or electromechanical devices, leakage currents associated with such devices are almost negligible. However, mechanical and electromechanical devices are inherently less reliable than their semiconductor counterparts. Unfortunately, the leakage currents associated with semiconductor switches, although small, are not so small that semiconductor switches may readily be used in a circuit to develop and thereafter maintain a DC control voltage across a capacitor for a time long enough to accommodate most television viewing situations. In addition, because the voltage storage capacitor is necessarily coupled to the circuit which is to be controlled in response to the DC control signal stored by the capacitor, leakage currents between the controlled circuit and the storage capacitor will tend to change the DC control voltage. Therefore, there is a requirement in the art for a circuit including a storage capacitor for generating and maintaining a DC control voltage.